Digital image reproduction systems are well-known. These systems typically include a digital document generator that may be coupled to the reproduction system directly or through a computer network. Digital document generators include computers, scanners, or other devices that store or permit a user to define content for a digital document. The digital data are provided to a print engine so the controller of the engine may control the process. The reproduction system also includes a photoreceptor belt or drum that provides a rotating surface for the exposure, development, and transfer of a latent image that corresponds to the digital document.
The latent image exposure begins with the charging of a portion of the photoreceptor belt at a charging station. The charged portion of the belt is advanced through an imaging/exposure station, where the data digital are provided as a signal to a raster output scanner. The raster output scanner selectively discharges the charged portion of the photoreceptor belt to form the latent image in correspondence with the document digital data. Development of the latent image occurs with the advancing of the photoreceptor belt to a development station where toner is attracted to the exposed latent image on the photoreceptor belt. More than one development station may be used for the development of color images so that different color toner materials may be applied to the latent image. Once the latent image is developed, the belt rotates to a transfer station where the toner on the latent image contacts a sheet medium, such as a sheet of paper. Typically, a corona generating device generates a charge on the backside of the sheet medium so the toner particles are attracted to the sheet medium and migrate from the latent image to the sheet medium. A detack unit removes the sheet medium from the photoreceptor belt and the belt moves through a cleaning station to remove the residual toner particles so that portion of the belt may be used for development of another latent image. The sheet medium impregnated with toner particles moves to a fuser station where fuser and pressure rollers permanently fuse the toner particles to the sheet medium. The sheet medium is then directed to a catch tray for the accumulation of sheets bearing the images of the digital documents sent to the reproduction system.
To provide data for the control of this reproduction process, one or more densitometers or enhanced toner area coverage (ETAC) sensors may be provided after the development station(s) to measure the developed mass of toner applied to a unit area, sometimes called developed mass per unit area (DMA), on the photoreceptor belt or drum. The ETAC sensor includes one or more light emitting diodes (LEDs) for emitting light at a particular wavelength, which is preferably in the infrared range. The LEDs of the ETAC sensor are oriented at a particular angle with respect to the photoreceptor belt so that the emitted light is reflected by the toner on the photoreceptor belt and one or more photodetectors are located at the reflection angle to receive the light reflected from the photoreceptor belt. Typically, the latent image includes a toner control patch so the emitted light impinges on an area having toner to produce the toner density measurements. The voltage signal generated by a photodetector may be used to determine the DMA for the application of toner to the photoreceptor belt or drum.
The photodetectors are located in the area of reflected light so that one or more of the photodetectors receive specular light reflected from the photoreceptor. Other photodetectors are located so that they receive diffuse light reflected from the applied toner. The photodetectors generate a voltage signal that corresponds to the amount of light received by the photodetector. Thus, the photodetectors provide a specular measurement and a diffuse measurement. The specular measurement refers to light reflected by bare photoreceptor within the toner patch that presents a mirror surface to the emitted light, while the diffuse measurement refers to light reflected by the toner patch that is uneven and diffuses the emitted light from the LEDs. Both signals are important for reproduction control because the specular measurement is self-calibrating with LED intensity variations but saturates at typical solid area masses while the diffuse measurement remains sensitive to toner mass as it increases but is altered by LED intensity variations. Consequently, the specular signal has good signal to noise ratio characteristics for low DMA levels, while the diffuse signal has good signal to noise ratio characteristics for high DMA levels.
A controller for a print engine in a digital reproduction system may use the specular and diffuse measurements received from the ETAC sensors to detect degradation in the quality of the images being reproduced by the system. One commonly encountered image defect is a class of defects known as banding defects. These defects produce lines, streaks, or bands extending across the image. These defects may occur in the direction of the image production process or perpendicular to the process direction. The effects of the defects may appear in images periodically or non-periodically. The banding defects are typically the result of worn or damaged parts, foreign matter, electrical malfunctions, vibrations, or component misalignment in the print engine and associated feed mechanisms and controls.
In published U.S. Patent Application 2003/0142985 entitled Automated Banding Defect Analysis And Repair For Document Processing Systems filed on Jan. 30, 2002, which is commonly owned by the assignee of this patent and the entire disclosure of which is hereby expressly incorporated herein by reference in its entirety, a system for detecting banding errors and identifying a remedial service procedure is disclosed. That system uses a number of diagnostic techniques to detect banding errors and identify a cause for the banding error. Some of the remedial measures may be performed by the operator or other on-site personnel, while some remedial measures must be performed by service technicians. By identifying remedial measures that may be performed by an operator or other on-site personnel, downtime is reduced and unnecessary service calls are avoided.
One issue with the analysis and repair system is the cost of repair. That is, the remedial measures identified by the system typically require replacement of parts or adjustment of existing parts. These procedures require some human intervention and some downtime for the reproduction machine. The remedial measures, in some cases, are overkill because the cost of new part may not be warranted since the image defect may be tolerable. Consequently, the analysis and repair system may result in the removal and discarding of reproduction machine parts before they have lost their full operational effectiveness.